MaineJay

Looks like a prolonged event involving a slow moving long wave, with several shortwaves occasionally energizing it. The blocking continues, and even puts me in the "warm" sector largely. the downside is the rain.

TROUGHING WILL BE REINFORCED IN THE EASTERN THIRD/HALF OF THELOWER 48 NEXT WEEK WHILE RIDGING OVER THE WEST HOLDS AN UPPER LOWOFF THE COAST. THE MODELS HAVE STRUGGLED WITH THE TRACK OF THEUPPER LOW AND NORTHERN STREAM FLOW OUT OF CANADA BUT SEEM TO BEHONING IN ON A BETTER CONSENSUS COMPARED TO 24 HRS AGO. THIS WOULDKEEP THE UPPER LOW OFF THE CALIFORNIA COAST THROUGH THE WEEK UNTILNEXT WEEKEND WHEN IT MAY LIFT NORTHEASTWARD TOWARD THE CA/ORBORDER. A BLEND OF THE RECENT GUIDANCE OFFERED A GOOD STARTINGPOINT DESPITE SOME DIFFERENCES IN STRENGTH/TIMING OF A FEW OF THEEASTERN SYSTEMS IN THE LONGWAVE TROUGH PATTERN. COULD NOTNECESSARILY RULE ANY ONE RUN OUT GIVEN THE MULTI-DAY SHUFFLINGS.

...WEATHER/THREATS HIGHLIGHTS...

EXITING LEAD UPPER/SFC LOW WILL SPREAD MODEST RAINS THROUGH THEMID-ATLANTIC EARLY IN THE PERIOD WITH GENERALLY LIGHTER AMOUNTSINTO NEW ENGLAND AS THE LOW RIDES UP I-95 INTO MAINE ON THURSDAY.TRAILING SYSTEMS WILL SPREAD LIGHT RAIN OR MAYBE SOME WET SNOWTHROUGH THE GREAT LAKES AS WEAKER SHORTWAVES SLIP SOUTHEASTWARD INTHE BROAD CYCLONIC FLOW. TEMPERATURES WILL BE BELOW AVERAGE WITHTHE EXCEPTION OF MAINE.

IN THE WEST, UPPER RIDGING WILL ENSURE WELL ABOVE AVERAGETEMPERATURES (+10 TO +20F ABOVE CLIMO) WITH SOME RECORDS POSSIBLE.PRECIPITATION MAY HOLD OFFSHORE UNTIL NEXT WEEKEND (OR AT LEASTLATE FRIDAY) WITH THE ENSEMBLE CONSENSUS MOSTLY INTO COASTALOREGON BUT ALSO INTO THE SIERRAS AS THE COLD FRONT SHOULD FINALLYCOME ASHORE.

LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...Model guidance remains consistent with ridging to the N of upperlow/trof forecast to slowly churn across the Srn CONUS thruearly next week. The gradual warm up will continue into earlynext week...with some readings in the 60s likely Mon and Tue.Multi-model consensus is a little on the cool side...mainlybecause the recent colder than normal wx is dragging the biascorrection away from warm temps. I have added a little more MOSguidance to the blend to try and offset that effect thru Tue.

By late Tue thru Wed model guidance brings in the next round ofprecip. That slow moving upper low lift NEwd...and with PWATvalues forecast to be above normal for this time of year weshould see a pretty widespread rainfall. While it will not be awarm rain...it will be mild enough for mainly rain except forthe highest elevations of the forecast area. The combination ofgradual snow melt in the milder temps plus rainfall will allowriver flows to increase thru next week. Looking at ensemble flowguidance...that is enough to push a handful of headwater gaugelocations above flood stage late in the week. So it will besomething to keep an eye on.

So I'm not sure how great a winter threat this is, but in looking through the EPS members, there is support for some type of system moving through. I believe it's separate from the previous thread from Poc and the subsequent one from UTS. At least I hope so.

Everything from a western great Lakes track, to something sliding off the SE coast, and everything in between is represented.

Pattern: Persistent pattern of the past few days continues earlythis morning with vortex centered just north of Hudson Bay...anotherover eastern Asia....with ridging just off the west coast of NorthAmerica with early morning water vapor imagery revealing ashortwave helping re-establish troughing over the southwesternUnited States. Deterministic/ensemble guidance remains in strongagreement in maintaining this pattern through the comingweekend. Beyond this...there continues to be strong signals fora significant pattern change with Pacific ridge retrogradingwest and building...which allows significant troughing to takeshape over western North America with a weakening/eastward shiftof the Hudson Bay vortex. This favors building heights overeastern North America with a Bermuda high setup directing deepsouthwesterly flow along the eastern seaboard. The flow remainsprogressive through this period with no downstream blocking...so even this warm up does not look to remain with us for verylong. Primary forecast concerns through the long term forecastperiod include potential weekend snowfall...then the focus turnsto deep moisture plume along the eastern flank of developinglongwave trough and how much precipitation potential we havefrom this feature.

The Details...

Monday-Monday Night: High pressure will be centered just east of theregion to begin Monday with warm advection and top down saturationrapidly setting in as the day progresses. Thus...expect an increasein clouds with rain showers approaching from the south and westtowards evening. T9s moving to around the freezing mark should allow40s for areas outside of the mountains. Showery precipitation looksto arrive Monday night and will likely take the form of rain formost location...with the potential for a mix at the onset over themountains although even here temperatures aloft warm rapidly so thethreat will more likely be from pockets of trapped low levelcold air potentially leading to some freezing rain. Mild nightwith a non- diurnal trend likely /temps slowly rising throughthe night/ with 30s and potentially some 40s south.

Tuesday - Wednesday: Substantial run to run and model to modeldifferences develop by this time range with the EC/GFS/GGEM /andtheir ensemble counterparts/ all favoring a western trough...but very different evolutions along the eastern flank of thisfeature...reducing confidence. There is good confidence intemperatures moving well above normal for Tuesday and likelyWednesday with T8s at or above 10C. PWATs also increasedramatically as the Gulf of Mexico opens up. Looking at both theEC EPS and the GEFS...there is certainly enough overlap inguidance thinking to keep chance PoPs through this period...withthe greatest precipitation potential favored west as currentindications suggest that the best heavy rainfall signal will bewest of our region. Certainly something to keep an eye on forpotential hydro concerns. Otherwise...story will betemperatures...with good agreement that T9s move to near orabove 10C Tuesday...with some potential to be warmer still onWednesday. While this suggests the potential for 60s...have tobe careful with precipitation potential as well as decreasingconfidence... particularly on Wednesday. Still...boosting highsfor both days with 40s north and 50s south is a good startingpoint that can be adjusted as confidence grows.

I thought it might make sense to start a thread to document the upcoming cold. Not sure of the end date, suggestions welcomed.

EPS

Also, since there might be some waves that move along the edge of the cold, feel free to discuss potential for snow if there's no thread for it.

GYX

QUOTE

Beyond Christmas we begin to see signs of very cold air poisedto drop into the area. The core of a very cold Arctic low willbe sitting over south central Canada and southwest Hudson Bay,and the departure of the Christmas storm will begin to draw thiscold air southeastward into northern New England. With the freshcoat of snow across New England, expect this cold air mass toencounter little resistance in heading this way, reaching ourarea with full potency. The leading edge of this cold airarrives on Tuesday, with a stiff west northwest windaccompanying it. 925MB temperatures of -20C to -30C suggestsurface temperatures falling below zero Tuesday evening in thenorthern and eastern part of the area in spite of the low levelmixing. Very dry air mass, fresh snow pack, and some of thelongest nights of the year should allow nighttime temperaturesto fall below zero area wide, and to -20F or colder in thenorthern valleys Wednesday night and Thursday night. Daytimehighs may not rise above zero in northern and eastern areas fora few days! It is likely that a wind chill headline of some sort(Advisory or Warning) will be needed for part of the area whenthis cold air arrives, most likely in the Tuesday to Wednesdaytime period when winds are still fairly strong.

Last 5 ECMWF runs. Last run is valid at hour 114, this is when models start to become a bit more reliable. Unfortunately, there appears to be subtle phasing details which have significant sensible weather effects. So I'll be watching for the potential for stream interactions. Or even a brief cutting off.